1. Field of the Invention
This invention generally relates to a pixel structure of a liquid crystal display, and more particularly to a pixel structure of a liquid crystal display with high aperture ratio and low coupling ratio.
2. Description of the Related Art
Accompanying with the improvement of electronic technology, especially the popularity of portable electronic products in daily life, there is an increased demand for light, compact and low power consumption display devices. Because a liquid crystal display has the merits of low power consumption, compact and light, it is suitable for this kind of electronic products and even gradually replaces conventional cathode ray tube (CRT) devices.
Because the pixel aperture ratio is an important factor that has an effect on the characteristics of liquid crystal displays, several kinds of pixel structures have been proposed so far to increase the pixel aperture ratio. Referring to FIGS. 1 and 2, FIG. 1 shows a plane view of a conventional pixel structure with high aperture ratio while FIG. 2 shows a cross-sectional view taken along the line II-II′ of FIG. 1. The pixel structure 9 includes a gate line 91 and a storage line 92 formed parallel in a row; a data line 93 perpendicular to the gate line 91 and the storage line 92, wherein the storage line 92 has a first part 92a serving as a common line and a second part 92b serving as a storage capacitor and the width of the second part 92b is larger than that of the first part 92a. The gate line 91 and the data line 93 define a pixel region.
A thin film transistor 95 is disposed adjacent to an intersection of the gate line 91 and the data line 93 and includes a gate electrode 91a extended from the gate line 91, a semiconductor layer 951 formed on the upper of the gate electrode 91a with sandwiching an insulating layer 98, as shown in FIG. 2. A source electrode 953 and a drain electrode 952 overlap with both side portions of the semiconductor layer 951, respectively. An organic insulating layer 97 is formed over the pixel region and a pixel electrode 96 is further stacked thereon. A contact hole 99 is provided through the organic insulating layer 97 so as to electrically connect the pixel electrode 96 to the source electrode 953, wherein the pixel electrode 96 overlaps with portions of the gate line 91 and the data line 93 respectively, thereby increasing the aperture ratio of the pixel structure 9.
However, in the above pixel structure 9, an organic insulating layer 97 is disposed to decrease the parasitic capacitance Cpd existed between the pixel electrode 96 and the data line 93 thereby reducing crosstalk. Referring to FIG. 3, it shows the connection between capacitors in a pixel region. With reference to this drawing, the coupling ratio in a single pixel region can be obtained as (Cpd1+Cpd2)/[(Cpd1+Cpd2)+Cst+Clc+(Cgs+Cpg)], where (Cpd1+Cpd2) is the parasitic capacitance induced by the overlapping of a pixel electrode with data lines of this pixel region, Cst is the storage capacitance of the pixel region, Clc is the capacitance of liquid crystal unit, Cgs is the parasitic capacitance between a gate electrode and a source electrode of the thin film transistor, Cpg is the capacitance between a pixel electrode and a gate electrode of the thin film transistor. If the coupling ratio in a single pixel region becomes smaller, the crosstalk becomes smaller. And according to the above equation, the coupling ratio can be reduced by decreasing the value of (Cpd1+Cpd2) or by increasing the value of Cst.
Although the parasitic capacitance Cpd between the pixel electrode 96 and the data line 93 can be reduced by disposing an organic insulating layer 97 in the pixel structure 9 as shown in FIGS. 1 and 2, the storage capacitance Cst between the pixel electrode 96 and the second part 92b of the storage line 92 will also be reduced simultaneously. Therefore, the coupling ratio in a single pixel region is not able to be effectively reduced.
Therefore, the present invention further provides a pixel structure of a liquid crystal display which can increase the aperture ratio of a pixel structure and reduce the coupling ratio in a single pixel region.